1. Field of the Invention
The present invention relates to an image processor, and more particularly, to an encoder in a predictive coding system.
2. Description of the Background Art
FIG. 8 is a block diagram showing a configuration of an encoder 101 in a predictive coding system. The encoder 101 includes a quantization unit 102, a prediction unit 103, and an encoding unit 104. Data D100 is inputted from a preceding processing block (not shown) to the quantization unit 102. The quantization unit 102 quantizes the data D100, so as to output data D101. The data D101 is inputted from the quantization unit 102 to the prediction unit 103. Meanwhile, data which was previously processed has been inputted to the prediction unit 103 as prediction data D102. The prediction unit 103 outputs a difference value between the data D101 and the prediction data D102 as data D103. The data D103 is inputted from the prediction unit 103 to the encoding unit 104. The encoding unit 104 performs entropy coding on the data D103, so as to output coded data D104.
Microsoft Corporation has recently proposed HD Photo (or JPEG XR) as a still image file format that offers higher image quality than JPEG while requiring more simple circuit configuration and computation than JPEG 2000.
FIG. 9 is a block diagram showing a configuration of an encoder 201 for HD Photo. The encoder 201 includes a color conversion unit 202, a pre-filter 203, a frequency transform unit 204, a quantization unit 205, a prediction unit 206, and an encoding unit 207.
A pixel signal D200 of RGB color space is inputted from an imaging element, such as a CCD or CMOS image sensor, to the color conversion unit 202. The color conversion unit 202 converts the pixel signal D200 into a pixel signal D201 of, for example, YUV color space, and outputs the same. The pixel signal D201 is inputted from the color conversion unit 202 to the pre-filter 203. The pre-filter 203 performs prefiltering to reduce block artifacts on the pixel signal D201, and outputs a pixel signal D202. The pixel signal D202 is inputted from the pre-filter 203 to the frequency transform unit 204. The frequency transform unit 204 performs predetermined frequency transform (PCT: HD Photo Core Transform) on the pixel signal D202, and outputs data D203 after frequency transform. In HD Photo, the data D203 includes highpass, lowpass, and direct current components.
The data D203 is inputted from the frequency transform unit 204 to the quantization unit 205. The quantization unit 205 quantizes the data D203, so as to output data D204. The data D204 is inputted from the quantization unit 205 to the prediction unit 206. Meanwhile, data which was previously processed has been inputted to the prediction unit 206 as prediction data. The prediction unit 206 outputs a difference value between the data D204 and the prediction data as data D205. The data D205 is inputted from the prediction unit 206 to the encoding unit 207. The encoding unit 207 performs entropy coding on the data D205, so as to output coded data D206.
The details of HD Photo are disclosed in, for example, “HD Photo—Photographic Still Image File Format”, [online], 7 Nov. 2006, Microsoft Corporation, [searched in the Internet on 10 Oct. 2007], <URL: http://www.microsoft.com/whdc/xps/hdphotodpk.mspx>. The details of JPEG XR are disclosed in, for example, “Coding of Still Pictures -JBIG JPEG”, [online], 19 Dec. 2007, ISO/IEC JTC 1/SC 29/WG1 N 4392, [searched in the Internet on 4 Mar. 2008], <URL: http://www.itscj.ipsj.orjp/sc29/open/29view/29n9026t.doc>.
In the encoders 101 and 201 shown in FIGS. 8 and 9, the values of data after prediction (data D103 and D205) inputted to the encoding units 104 and 207 are preferably as small as possible, in order that an amount of code of the coded data D104 and D206 outputted from the encoding units 104 and 207 is reduced.